Conventional digital phase locking systems such as frame synchronizers and time-base correctors are described in U.S. Pat. Nos. 3,909,839 entitled "PHASE LOCKING SYSTEM FOR TELEVISION SIGNALS USING DIGITAL MEMORY TECHNIQUES" and 4,007,486 entitled "PHASE LOCKING SYSTEM FOR TELEVISION SIGNALS USING A DIGITAL MEMORY TECHNIQUE". In such conventional digital phase locking systems, an input television video signal is coded and then stored in digital memory means in response to a first clock pulse train synchronized with the input television video signal. The stored signal is read out in response to a second clock pulse train synchronized with a reference signal.
In television systems such as the NTSC, PAL and SECAM systems used in present television broadcasting, an interlaced scanning system is adopted. In the NTSC system, for example, 30 pictures are transmitted per second, and a frame representing one picture consists of two, odd-and even-numbered fields, differing by one half of one horizontal scanning line with respect to the scanning position. Further, the color-subcarrier phase, in two adjacent frames, is opposite to each other. Assume, that the digital memory capacity, utilized in prior art systems, corresponds to one field. Then, if the memory content is read out duplicatively, i.e., read out before it has been replaced by new data of a following field (duplicative read-out), or vice versa (duplicative write-in), the result is a phase-shaft on the read-out signal wherein the vertical synchronizing signal is phase-shifted by one half (1/2H) of one horizontal scanning period (1H). In addition, in the case where memory capacity corresponds to one frame, the duplicative read-out or duplicative write-in will give rise to another phase-shift wherein the horizontal synchronizing signal is phase-shifted by one half (about 140 nanoseconds) of one cycle period of the color subcarrier. These phase-shifts in the read-out signal result in visual effects on the reproduced picture in that reproduced scanning lines are visable in the vertical or horizontal direction.
The rate of occurrence of such undesired duplicative read-out or write-in is proportional to the frequency difference between the synchronizing signals contained in the input and the reference signals, and inversely proportional to the memory capacity when the input signal does not fluctuate in frequency. Assuming that the frequency difference with respect to the subcarrier frequency of 3.58 MHz is 10 Hz and the memory capacity is one field, the rate of occurrence is approximately once every 1.66 hours. Such an occurrence is negligible as a practical matter.
Conversely, a reproduced signal from a video tape recorder (VTR) contains frequency fluctuations, i.e., a time-base error. To compensate for the time-base error, a time base corrector (TBC) has been employed, which has a compensating range of several horizontal scanning periods. When utilizing a TBC the VTR is driven by the reference signal supplied from the TBC so that the phase of the VTR-reproduced signal is within a predetermined phase-range with respect to the phase of the reference signal.
In the case where a reference signal can not be supplied to the VTR, the VTR reproduces a video signal independent of the reference signal. The independently reproduced video signal is supplied directly to a frame synchronizer. The frame synchronizer comprises a clock pulse generator for producing a write-in clock pulse train following the time-base fluctuation and memory means having a capacity of at least one field, to compensate for the time-base error and to phase-lock the reproduced signal to the reference signal. However, the VTR-reproduced signal contains the time-base error. When the phase of the reproduced signal approaches that of the reference signal, duplicative read-out and duplicative write-in become frequent making frequent visual shifts in the reproduced picture on the picture tube.
It is, therefore, an object of this invention to provide an improved phase locking system capable of phase-locking an independently reproduced video signal from the VTR to the reference signal without causing visual shifts in the reproduced picture on the picture tube.